multiple improvements, use slim-liquid

* More consistent module structure
* Explicitly require activesupport where needed
* Use slim-liquid to improve slim/liquid interplay

Requires https://github.com/slim-template/slim-liquid and slim master.

This allows to write things like

~~~
% if ...
   tags
~~~

which generates Slim code. I still have to document https://github.com/slim-template/slim-liquid